Typical wheelchairs have two large diameter rear driving wheels positioned beneath the seat portion of the chair and smaller diameter pivoting castor wheels (four to eight inches in size) positioned in front of the rear wheels. However, the problem with these current wheelchairs is that the smaller castors provide reduced stability and are not usable off-road. Problems encountered by a user attempting to utilize a typical wheelchair off-road include: instability caused by a high center of gravity, causing the wheelchair to topple backwards or sideways when negotiating steep grades; the wheels becoming stuck while traversing uneven surfaces; and/or the wheelchair is unable to traverse over objects in its path due to instability or inadequate clearance. Further, if the wheelchair is configured to have more clearance, a user's legs will not fit under tables.
To address the uphill stability/tipping concerns, some wheelchairs are equipped with anti-tippers. Anti-tippers are additional little wheels designed to prevent a wheelchair from tipping over backward. But anti-tippers restrict a wheelchair's ability to go over obstacles because they can get caught on the obstacle. In addition, adjusting, engaging, or disengaging the anti-tippers is extremely difficult for a seated wheelchair user because the anti-tippers can not be reached easily by the user.
Even when utilizing sport model wheelchair types (i.e., where the chair is configured such that the user's knees are bent upward and the feet rest nearly in the same plane as the buttocks), a wheelchair user is faced with similar problems when attempting to traverse off-road and in a steep incline situation.